ramips: add support for ipTIME AX2002MESH

ramips: add support for ipTIME AX2002MESH

ipTIME AX2002MESH is an 802.11ax (Wi-Fi 6) router based on
MediaTek MT7621 SoC.

Specifications:
- SoC: MediaTek MT7621, MIPS 1004Kc, 880 MHz
- RAM: 128 MiB
- Flash: NAND 128 MiB (ESMT PSU1GA30DT)
- WLAN: MediaTek MT7915E
  - 2.4 GHz: ax/b/g/n
  - 5 GHz: ac/ax/n
- Ethernet:
  - LAN: MediaTek MT7530, 2x 1GbE
  - WAN: MediaTek SoC built-in, 1x 1GbE
- UART: J4 header, 115200 8N1
  - Pinout: [3V3] [TX] [RX] [GND]
- Button: Reset
- LEDs: 6 LEDs
  - 1x CPU (Amber)
  - 1x WLAN 5GHz (Amber)
  - 1x WLAN 2.4GHz (Amber)
  - 2x LAN (Amber)
  - 1x WAN (Amber)
- Power: DC 12V/1A

MAC addresses:
| Interface | MAC               | Algorithm                 |
|:----------|:-----------------:|:--------------------------|
| WLAN 2.4G | 58:86:94:XX:XX:34 | label                     |
| WAN       | 58:86:94:XX:XX:35 | label + 1                 |
| LAN       | 58:86:94:XX:XX:37 | label + 3                 |
| WLAN 5G   | 5A:86:94:XX:XX:37 | label + 3 with LA bit set |
- The WLAN 2.4G MAC address (label) is stored in factory partition, 0x4

Installation:
- Stock web interface:
  1. Flash the **initramfs-factory** image through the web page.
  2. Boot into OpenWrt and perform sysupgrade with **sysupgrade** image.
- Recovery mode:
  1. Turn on the device and press the Reset button more than 10 seconds
     to enter recovery mode.
  2. Access the recovery web interface at 192.168.0.1/24 and flash the
     **initramfs-factory** image.
  3. Boot into OpenWrt and perform sysupgrade with **sysupgrade** image.

Signed-off-by: Myoung Ha, Ji <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20584
Signed-off-by: Hauke Mehrtens <[email protected]>